Fuel bundle designs for boiling water nuclear reactors are known. Such fuel bundle designs are fabricated in a standard fashion including a lower tie plate for supporting an upstanding matrix of fuel rods in side-by-side relation and permitting the inflow of water coolant into the fuel bundle. Most of the fuel rods of such a fuel bundle design extend from the supporting lower tie plate to an upper tie plate. This upper tie plate serves to maintain the fuel rods in upstanding side-by-side relation and to permit the exit of water and generated steam from the fuel bundle.
The fuel bundle is typically surrounded by a fuel bundle channel, which channel surrounds the lower tie plate, extends upwardly around the fuel rod matrix, and surrounds the upper tie plate. This fuel bundle channel isolates the flow path through the fuel bundle so that water and steam generated in the interior the fuel bundle are separate from the so-called core bypass region surrounding the fuel bundle. This core bypass region contains water moderator and occupies generally cruciform shaped volumes between the fuel channels into which control rods can penetrate for the absorption of thermal neutrons for the control of the nuclear reaction.
In operation of the boiling water nuclear reactor fuel bundles, liquid moderator--water--is introduced at the bottom of the fuel bundle through the lower tie plate. The water passes upwardly interior of the fuel bundle and performs two major functions. First, it moderates so-called fast or high velocity neutrons produced in the nuclear reaction to slow or thermalized neutrons needed to continue the nuclear fission reaction. Secondly, the water moderator generates the steam which is utilized for the generation of power.
It will be understood that the fuel rods interior of the fuel channels are long slender sealed tubes containing fissionable material and are flexible. If such fuel bundle were to be unrestrained, they would vibrate and even come into abrading contact with one another during the generation of steam. To restrain this tendency as well as maintain the fuel rods in their designed side-by-side spacing for efficient nuclear operation, so-called spacers are utilized. These spacers are placed at selected vertical intervals within the fuel bundle. Usually, seven evenly distributed fuel rod spacers are utilized in a fuel bundle having an overall length in the order of 160 inches. These spacers surround each individual fuel rod maintaining the precise designed spacing of the fuel rods along the entire length of the fuel bundle.
So-called part length rods have been introduced into this standard fuel bundle construction environment. These part length fuel rods extend from the lower tie plate only partially the distance to the upper tie plate. The fuel rods typically terminate underlying the upper tie plate so as to define an unoccupied vertical interval within the fuel bundle starting at the top of the part length fuel rod and extending to the upper tie plate. These part length fuel rods have many advantages, which advantages are summarized in Dix et al. U.S. Pat. No. 5,017,332 entitled Two Phase Pressure Drop Reduction BWR Assembly Design issued May 21, 1991.
One of the greatest single advantages of part length fuel rods is the reduction of pressure drop in the upper two phase region of the fuel bundle. Simply stated, in the upper two phase region of the fuel bundle, void volumes are defined between the ends of the part length fuel rods and the upper tie plate. With the omitted fuel rod volumes overlying the part length fuel rods in the upper two phase region of the fuel bundle, the two phase water/steam mixture present within the fuel bundle is freer to move in the vacated region. This being the case, the realized pressure drop in this region of the fuel bundle is lessened. While it is not the purpose of this disclosure to be a primer on the operation (and pressure drop variations) of nuclear fuel bundles, the reader can understand that reduced pressure drop can be an advantage.
Modern fuel bundles and the contained fuel rods also contain another limitation. Specifically, and during the operation of the fuel bundles, fission gas accumulates interior of the fuel rods of the fuel bundle. To fully understand this phenomenon, the construction of fuel rods should first be reviewed with the necessity of fission gas accumulation being summarized thereafter.
The discrete fuel rods within fuel bundles are typically sealed Zircalloy tubes. These sealed Zircalloy tubes contain the nuclear fuel in pellet format stacked typically in pellets occupying the almost the full diameter of the tubes. It has been found advantageous to first evacuate and thereafter fill the sealed tubes with an inert gas under pressure to provide better heat transfer between the fuel pellets and the enclosing zircalloy tubes as well as reduction of fission gas emission during the in service life of the fuel bundle.
When the fissionable materials interior of the sealed fuel rods are subjected to nuclear reaction, certain "daughter" fission gasses are produced. It is the primary purposed of the fuel rods--or cladding--around the fuel pellets to contain this gas.
Inert gas, usually helium, is added to the interior of sealed fuel rods containing fissionable material. Typically, a vacuum is first drawn on the sealed interior of the fuel rod. Substantially all oxygen and water is withdrawn from the interior of the fuel rod. Thereafter, helium under pressure is added. This helium under pressure is a good heat transfer medium - serving to keep the fuel pellets relatively cool and causing heat to be conducted away from the pellets for the production of energy producing steam. At the same time, the fuel pellets during their in service life are maintained in a cooler state where less "daughter" fission gases are produced.
It is important that helium added to the fuel rod be provided with a sufficient plenum volume. This plenum volume maintains the helium within the desired density and pressure range between the fuel pellets and cladding during the full in service life of the fuel rod within the fuel bundle. At the beginning of inservice life, the pressurized helium provides a pressure interior of the fuel rod that balances the pressure exterior of the fuel rod. At the end of in service life, sufficient expansion volume is present to prevent the cladding of the fuel rod from becoming over pressurized with resultant gas leakage.
Calculation for the amount of expansion volume required for the so-called plenum region of any fuel rod in any fuel bundle design is a known art. Over simplified, the calculation of this required plenum volume is a function of the fuel mixture utilized, the energy extracted from the total operation of the fuel rod at the end of life of the fuel bundle, the residence time of the fuel bundle in the reactor, exposure to the various fluxes present within the reactor and other factors which results in a fission gas pressure whose value must be limited to prevent excessive strain in the fuel rod tubing.
It will be understood that normally fuel bundles have a length on the order of 160 inches. Of this total length of about 160 inches, some 150 inches of length of the fuel bundles can be occupied by active fuel. As a practical matter, active fuel is never placed in a fuel bundle at lengths greater than 150 inches because the resultant radiation exposure--typically at the top portion of the core--is undesirable with modern reactor design.
Unfortunately, and even with 150 inches available for active fuel, certain fuel designs require excessive plenum volumes. That is to say that even with a fuel rod that is 160 inches long and loaded fuel potentially occupying 150 inches of that total vertical dimension, the remain 10 inches of space within the fuel rod do not define a sufficient plenum volume. By way of example, designs are known in which only 144 inches of the total available 150 inches can be loaded with fuel. The remaining volume is required for the gas plenum of the fuel rods.